Rotary drum filters are used in the pulp and paper industry to filter liquid out of pulp. The filters include a cylindrically shaped drum that is typically constructed of metallic and/or fiberglass materials. In operation, the drum is partially submerged in a vat containing a paper pulp stock. The outer surface of the drum is wrapped with a metal or plastic mesh known as a face wire which acts as a filtering medium that passes liquid as the drum is rotated. Pulp is retained on, and subsequently removed from, the outer surface of the drum.
The drum includes a cylindrical shell that is supported on the inside by flyrings and is closed at each end by end supports. Longitudinal grids are installed perpendicular to the outside surface or deck of the shell in a predetermined spaced relationship. The longitudinal divider grids or rib elements extend along the entire length of the deck and define drain compartments that empty into vacuum shut off sections, referred to as buckets or ports, located at the periphery of the deck.
In some filters, referred to as “open channel filters”, the longitudinal grids are covered with a backing wire that forms the outer surface of the drum. This backing wire acts as the support for the face wire. Liquid can pass through the spaces provided between the backing wire while pulp is retained on the surface of the wire. In other filters, referred to as “corrugated deck filters”, an example of which is disclosed in U.S. Pat. No. 3,837,499 (the entire content of which is expressly incorporated hereinto by reference), the longitudinal grids are covered with a corrugated material that forms the outer surface of the drum. The corrugated material includes drain slits to allow passage of filtrate from the surface of the drum to the drain compartments located beneath the deck and defined by the longitudinal grids. The deck design of the '499 patent has one drain slit per port, but three or more drainage compartments beneath the deck. Openings in the longitudinal grids within the port, known as intermediate grids, have openings to allow filtrate to be distributed to the other compartments.
Another drum structure for a rotary drum filter is provided in U.S. Pat. No. 5,308,488 (the entire content of which is expressly incorporated hereinto by reference) and includes longitudinal grids which are also covered with a corrugated material that forms the outer surface of the drum. As in the '499 patent, the corrugated material includes drain slits to allow passage of filtrate from the surface of the drum to the drain compartments located beneath the deck and defined by the longitudinal grids. The deck design of the '488 patent, however, has multiple drain slits per port, with each drain slit corresponding to a single drain channel. Additionally, each drain channel has one distinct deck panel section.
While such prior drum structures are satisfactory for their intended purposes, some improvements are still desired. For example, it would especially be desirable if the improved filtering efficiencies of the deck structure according to the U.S. '488 patent could be improved by removal of the deck fastening hardware on the drum surface and/or providing such improved structure with the lower cost manufacturing and installation features of the deck structure according to the U.S. '499 patent. It is towards providing such improvements that the present invention is directed.
Broadly, the present invention is embodied in a drum structure for corrugated type rotary filters having relatively high filtering efficiency and relatively low manufacturing and/or installation costs. More specifically, according to the present invention, a corrugated deck section is rigidly connected to a rotary drum structure by means of solid divider grids which establish a drain channel. The drain channel is subdivided into respective compartments by means of intermediate solid grids which are unconnected to the corrugated deck, but provide subjacent radial support thereto. Plug dams are provided in the corrugation spaces between the upper edge of the intermediate grids and the lower corrugated deck surface so as to prevent substantially filtrate flow between individual compartments. Drainage slits are provided in the corrugated deck in alignment with each of the compartments. Filtrate may thus flow through the drainage slits and into a respective one of the compartments where it can then subsequently be drained transversely and longitudinally into a respective port associated with the drum filter.
These and other aspects and advantages will become more apparent after careful consideration is given to the following detailed description of the preferred exemplary embodiments thereof.